Surface treatments have been applied to impart corrosion resistance to various metallic bases. In particular, a zinc phosphate treatment has been generally used as a metallic base in automobile construction. However, this zinc phosphate treatment has a problem such that a large amount of sludge is generated as a by-product. Therefore, there is a demand for a next-generation of surface treatment that does not use zinc phosphate. As one of these, a surface treatment that employs a surface treatment agent that contains zirconium, titanium and hafnium (hereinafter, referred to as “zirconium system surface treatment agent”) has been studied.
Incidentally, the metallic base used in automobile construction and is required to have high corrosion resistance is usually subjected to a cationic electrodeposition coating after the surface treatment. As a reason why the cationic electrodeposition coating is employed, in addition to the coating film obtained by the cationic electrodeposition coating being excellent in the corrosion resistance, it is important for the cationic electrodeposition coating to have the property that the coating can be applied to the four corners of an automobile body that has a complicated shape, that is, the cationic electrodeposition coating has so-called “throwing power”.
As it is, when the cationic electrodeposition coating is applied to a metallic base that has been surface-treated with the zirconium system surface treatment agent, there are cases where a sufficient effect may not be obtained in the throwing power. Thus, in the case where the cationic electrodeposition coating is applied, when the throwing power is not sufficient, sufficient corrosion resistance is difficult to obtain.
In Patent Document 1, for example, a chemical conversion treatment liquid for metal surfaces, which contains at least one compound selected from an aqueous titanium compound and an aqueous zirconium compound, and an organic compound having a plurality of functional groups as a stabilizer is described, and as the organic compound, for example, a compound having a plurality of carboxylic groups such as lactic acid can be used. However, in Patent Document 1, while the coating adhesiveness between the surface of the metallic base that was treated with the chemical conversion liquid for metallic surface and an electrodeposition coating film and the corrosion resistance are described, the electrodeposition coatability (smoothness and the throwing power) is not described.
In Patent Document 2, a zirconium system surface treatment agent having an electrophilic reactive functional group is described. However, a surface treatment agent described in Patent Document 2 is not a metal surface treatment agent (a so-called “reactive surface treatment agent”) of a type in which a film is formed by a pH variation caused by etching on a surface of metallic base but is used as a metal surface treatment agent (so-called “coating type surface treatment agent”) of a type in which a film is formed by coating on a surface of metallic base and by drying. Further, in Patent Document 2, since there is no description of the electrodeposition coating of the metallic base that is surface treated with the metal surface treatment agent, it goes without saying that neither the electrodeposition coatability (smoothness and throwing power) is described.
Patent Document 3 describes that excellent throwing power can be developed when the cationic electrodeposition coating is applied to a metallic base that has been surface-treated with a metal surface treatment agent for cationic electrodeposition coating containing zirconium ion and tin ion. Further, in Patent Document 3, there is a description for the effect that benzotriazole may have if added to the metal surface treatment agent for the cationic electrodeposition coating as a rustproof agent. However, the surface treatment agent described in Patent Document 3 may not develop satisfactory throwing power.
Patent Document 1: PCT International Publication No. WO2011/002040
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2001-329379
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2008-291345